272 
TYPHOID PEVEE DISTEICT OF COLUMBIA. 
Table giving the monthly average number of bacteria found in 1 cc. of Potomac drinldng 
water during 1886. 
Xumber of 
observa- 
tions. 
isse. 
January 2 
February - 4 
March 5 
April - 4 
May 3 
June - 2 
July 2 
August - - 1 
September 2 
October 3 
November 1 
December 2 
1S.S7. 
January 3 
Average. 
Rain fall 
(inches). 
3.774 
3.46 
2,536 
2.79 
1,210 
4.16 
1.521 
4.21 
1.069 
i • 1 1 
34S 
4.9S 
255 
S.42 
254 
i.as 
17S 
1.04 
75 
2.31 
115 
^.69 
967 
3.07 
SS2 
2.19 
“This anomalous condition is not so difficult of explanation. In 
the winter the water as it reaches the city is more or less turbid^ and, 
when shaken, clouds, composed of very minute particles, are seen. 
These will pass through orchnary filter paper, and when gathered 
together as in distilling water the residue is made up of reddish earth. 
This turbidity, most pronoimced in winter, gradually disappears 
toward summer, when the water becomes very clear and limpid. 
The number of bacteria varied with the change in turbidity, being 
highest when the suspended matter was most abundant. This fact 
impressed me so strongly after a number of observations that it 
became possible to anticipate cpiite accurately the number of bacteria 
present by looking at the water in the tube with the transmitted 
light.” 
“ It seemed reasonable to conclude that whatever agency brought 
the suspended earth also brought the bacteria and that the earth 
contained the bacteria. Throughout the vdnter of 1886 I noticed 
that after heavy rains the turbidity increased cpiite suddenly, this 
fluctuation, of course, producing a corresponding rise and fall in the 
number of bacteria. The rain, washing dovm the soil from the sur- 
face drained by the tributaries of the river, was thus the cause of the 
turbidity. But was there any relation between the rainfall and the 
number of bacteria? Through the kindness of the signal officer I 
obtained the data given in the third column of the table. Comparing 
the second and third columns, the relation is certainly not on the 
surface. The heaviest rains occurred in July, but the number of 
bacteria chd not rise perceptibly and no turbichty appeared. The 
only explanation which suggests itself is that which must be sought 
